Interpreting Unit Level UL 9540a Test Metrics
Unit-level UL 9540A metrics define gas generation, radiant heat flux, and deflagration limits needed to clear NFPA 855 unit separation mandates.
Gas Generation Rate
Meaning
Volumetric measurement of chemical byproducts produced within a sealed lithium-ion cell during operation or storage defines a degradation metric. The gas generation rate represents the rate at which volatile compounds are formed from electrolyte decomposition or impurity reactions. This rate stops being a reliable indicator of health once the cell pouch swells sufficiently to cause internal delamination and mechanical failure.
Chemical Origin
Electrolyte decomposition occurs when the cell operates outside its designated voltage window or at elevated temperatures. Inside the cell, the gas generation rate increases as the protective solid electrolyte interphase layer breaks down and exposes raw anode material to the organic solvents. For instance, carbon dioxide and carbon monoxide are released during high-voltage cathode oxidation, whilst hydrogen and ethylene are generated at the anode during formation cycles.
These chemical reactions consume active lithium, causing permanent capacity loss and accelerating cell aging.
Measurement Process
In-situ monitoring provides a continuous record of gas accumulation during charge and discharge cycling. Archimedes method is widely used to determine the gas generation rate by weighing the cell while it is submerged in a reference fluid. These measurements are taken at set intervals to establish the rate of volume change.
If the rate spikes unexpectedly, it indicates an accelerating thermal or chemical runaway precursor.
Safety Consequence
Excessive pressure buildup within a sealed pouch cell presents a severe risk of rupture.